Various types of microphone systems have been used in various applications through the years. Microphones in these systems typically receive acoustic energy and convert this acoustic energy into an electrical voltage. This voltage can be further processed for other applications or for other purposes. For example, in a hearing aid system the microphone may receive acoustic energy, and convert the acoustic energy to an electrical voltage. The voltage may be amplified or otherwise processed by an amplifier, or by other signal processing electronics circuitry, and then presented by a receiver as acoustic energy to a user or wearer of the hearing aid. To take another specific example, microphone systems in cellular phones typically receive sound energy, convert this energy into a voltage, and then this voltage can be further processed for use by other applications. Microphones are used in other applications and in other devices as well.
In such systems, the output impedance of the amplifier must be somehow matched to the input impedance of the receiver or other signal processing electronics circuitry, or signal degradation will occur. More specifically, the output impedance of the amplifier is usually high while the input impedance of the receiver is much lower and this mismatch causes signal attenuation unless corrected. Previous systems have often inserted a buffer to attempt to accomplish impedance matching.
Unfortunately, these previous approaches have suffered from several drawbacks. The buffers used in these systems typically used transistors. These transistors were electrically biased in such a way that the output resistance of the circuit was high and did not match the input impedance of the receiver. Because of the mismatch, distortion and degradation of the signal occurred. This was manifested by the end user or application not being able to recognize the signal.
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